Radiation detector, method for manufacturing radiation detector, and imaging method

This application claims the benefit of priority to Japanese Patent Application Number 2022-123533 filed on Aug. 2, 2022. The entire contents of the above-identified application are hereby incorporated by reference.

The disclosure relates to a radiation detector and an imaging method.

With the development of image processing techniques, various image diagnostic apparatuses are widely used also in the medical field. In a diagnostic apparatus using radiation such as X-rays, a radiation Flat Panel Detector (FPD) that can directly convert radiation transmitted through a body or an object into digital data is used. For example, JP 2018-63666 A discloses such an FPD.

The radiation detector used for such image diagnosis is required to acquire a more accurate image. An object of the disclosure is to provide a radiation detector that can more accurately acquire an image of a subject, a method for manufacturing the radiation detector, and an imaging method.

A radiation detector according to an embodiment of the disclosure includes a substrate, a plurality of pixels arranged on the substrate, the plurality of pixels each including a switching element and a photoelectric conversion element, a scintillator arranged to cover the photoelectric conversion element of each of the plurality of pixels, and a storage device storing inspection image data acquired by irradiating the plurality of pixels with visible light before forming the scintillator or a calibration parameter based on the inspection image data.

According to the disclosure, a radiation detector that can more accurately acquire an image of a subject, a method for manufacturing the radiation detector, and an imaging method can be provided.

An FPD type radiation detector is used as a substitute for an X-ray film, and a subject is captured typically at an equal magnification with respect to an imaging surface of a radiation detector. Therefore, the imaging surface of the FPD type radiation detector is significantly larger than that of an image sensor for a digital camera. For example, the imaging surface of the FPD type radiation detector has a size of 10 inches×10 inches or greater.

It is not easy to form an insulating layer and a semiconductor layer with a uniform thickness and to make an exposure condition of photolithography and a temperature condition of heat treatment the same, over such a large area. Thus, differences occur in sensitivity between pixels due to variations in the film thickness of such various layers and variations in the manufacturing condition, and this may result in brightness unevenness in a radiation image to be captured. In view of such a problem, the inventor of the present application has conceived a novel radiation detector, a method for manufacturing the radiation detector, and an imaging method.

Embodiments of the disclosure will be described below with reference to the drawings. The disclosure is not limited to the following embodiments, and appropriate design changes can be made within a scope that satisfies the configuration of the disclosure. Further, in the description below, the same reference signs may be used in common among the different drawings for portions having the same or similar functions, and descriptions of repetitions thereof may be omitted. Further, the configurations described in the embodiments and the modified examples may be combined or modified as appropriate within a range that does not depart from the gist of the disclosure. For ease of explanation, in the drawings referenced below, configurations may be simplified or schematically illustrated, or a portion of the components may be omitted. Further, dimensional ratios between components illustrated in the drawings are not necessarily indicative of actual dimensional ratios.

Structure of Radiation Detector

A radiation detector according to the disclosure is used for an X-ray photographing apparatus using radioactive rays such as X-rays, or an X-ray FPD to be used for X-ray photographing, for example.is a plan view of a radiation detectoraccording to the first embodiment, andillustrates a cross section of the radiation detectortaken along a line II-II in.

The radiation detectorincludes an active matrix substrate() and a scintillator. Additionally, the active matrix substrateincludes a substrateand a pixel arrayincluding a plurality of pixels. The pixel arrayis formed on the substrate.

The substrateincludes a first main surfaceand a second main surfacepositioned at the opposite side to the first main surface. The second main surfaceis a radiation incident surface of the radiation detector. The first main surfaceincludes a pixel regionin which the pixel arrayis arranged and a peripheral regionthat is located outside the pixel regionand that surrounds the pixel region

The substrateis preferably made of an insulating material that hardly absorbs radiation to be detected. For example, the substratemay be a glass substrate to be used for a liquid crystal display panel.

is a schematic circuit diagram illustrating an example of a circuit configuration of the pixel array. The pixel arrayincludes a plurality of pixelsone-dimensionally or two-dimensionally arrayed. In the present embodiment, the plurality of pixelsare two-dimensionally arranged in the row direction and the column direction. Each of the pixelsincludes a switching element and a photoelectric conversion element electrically connected to the switching element. The switching element is, for example, an active element such as an MIM element, a TFT or the like, and in the present embodiment, the pixelincludes a TFT. The TFTincludes, for example, an oxide semiconductor layer including at least one element selected from the group consisting of In, Ga, and Zn, or a Si-semiconductor layer. The oxide semiconductor layer and the Si semiconductor layer may have various types of crystallinity such as polycrystal, microcrystal, a c-axis orientation distribution or the like.

The photoelectric conversion element receives scintillation light emitted from a scintillator, which will be described later, and generates charges by photoelectric conversion. The photoelectric conversion element is, for example, an element including a semiconductor layer and having various structures that can separate a hole-electron pair generated by a photon incident on the semiconductor layer. In the present embodiment, the pixelincludes a photodiode. The photodiodeincludes, for example, an i-type Si semiconductor layer, and a p-type Si semiconductor layer and an n-type Si semiconductor layer that sandwich the i-type Si semiconductor layer. The pixelmay further include an amplifier circuit that amplifies charges generated in the photodiode.

The pixel arrayincludes a plurality of scanning linesand a plurality of data lines. For example, the gates of the TFTsof a plurality of pixelsarranged in the column direction are connected to one scanning line. In addition, the sources of the TFTsof the plurality of pixelsarranged in the row direction are connected to one data line.

In the pixel array, various insulating layers and interlayer insulating films are disposed between constituent elements that need to be electrically separated, such as the TFT, the photodiode, the scanning line, the data lineand the like. In, such insulating layers and interlayer insulating films are collectively illustrated as an insulator.

The scanning lineis electrically connected to a paddisposed in the peripheral regionof the first main surface. The data lineis electrically connected to a paddisposed in the peripheral regionof the first main surface

The radiation detectorfurther includes a scanning line drive unitand a charge detection unitwhich are a driver IC of the pixel array, and a storage device. The scanning line drive unitincludes substratesand terminalsindividually provided on the substrates, and drive circuits for sequentially selecting the plurality of scanning linesare formed on the substrates. When the terminalsare connected to the pad, a portion including at least the terminalsof the substrateis positioned in the peripheral regionand is supported by the substrate. The scanning line drive unitis connected to the scanning linesvia the terminalsand the pad, and is electrically connected to the TFTsof the plurality of pixels. Although the scanning line drive unitis divided into two or more substrates in the present embodiment, the scanning line drive unitmay be formed on one substrate.

Similarly, the charge detection unitincludes substratesand terminalsindividually provided on the substrates, and charge detection circuits for receiving charges accumulated in the photodiodesand converting the charges into electric signals are formed on the substrates. When the terminalsare connected to the pad, a portion including at least the terminalsof the substrateis positioned in the peripheral regionand is supported by the substrate. The charge detection unitis connected to the data linesvia the terminalsand the pad, and is electrically connected to the TFTsof the plurality of pixels. In the present embodiment, the charge detection unitis divided into two or more substrates, but the charge detection unitmay be formed on one substrate.

The storage devicestores inspection image data for defect inspection of the pixelsperformed on the active matrix substrateor a calibration parameter based on such inspection image data. As will be described later, the defect inspection, that is, the quality of each of the pixelsis determined by irradiating the plurality of pixelswith visible light before forming the scintillatorsin the pixel regionto acquire the inspection image data and analyzing the acquired inspection image data. A case where the calibration parameter is stored in the storage devicewill be described in a second embodiment.

Such inspection image data is luminance data proportional to the charges accumulated in the photodiodeof each of the pixels. When one of the TFTof the pixeland the photodiodedoes not operate correctly, the obtained luminance value is equal to or less than a predetermined value, and thus, the defect of the pixel is determined based on such a luminance value. On the other hand, such a luminance value of each pixel also would reflect variation in characteristics of each pixel. Therefore, the pixel data is stored in the storage deviceand is utilized to suppress the brightness unevenness when a subject is imaged by radiation using the radiation detector.

The storage devicemay be, for example, a nonvolatile memory such as an electrically erasable programmable read-only memory (EEPROM). The storage devicemay be an independent IC different from the scanning line drive unitand the charge detection unit, or may be incorporated in either the scanning line drive unitor the charge detection unit.

The scintillatoremits scintillation light when radiation transmitted through a body or an object is incident thereon. The scintillatorcovers the photodiodesthat are photoelectric conversion elements of the plurality of pixels. For example, the scintillatorhas a sheet shape, and is bonded to the plurality of pixelswith an adhesive layersuch as an OCA interposed therebetween. The scintillatormay be a vapor deposition film.

The scintillatoris made of a material corresponding to radiation to be used. The radiation may be X-rays, α-rays, γ-rays, or the like. X-rays are widely used for a medical or industrial radiation FPD. As the scintillatorthat detects X-rays, a single crystal or polycrystal material such as Thallium activated Cesium Iodide (Tl:CsI), Gadolinium OxySulfide (GOS) or the like can be used.

When radiation is detected by the radiation detector, radiation X transmitted through a body or an object is caused to be incident on the second main surfaceside of the substrate. The radiation X transmits through the substrateand the pixel arrayformed on the first main surface, and is incident on the scintillatorfrom the second main surfaceadjacent to the photodiodes. The radiation X incident on the scintillatorexcites a substance constituting the scintillator, and scintillation light is emitted from the scintillator. The photodiodedetects the generated scintillation light and generates charges by photoelectric conversion. The charges generated by the photodiodein each pixelare converted into an electric signal by the charge detection unitin a reading order controlled by the scanning line drive unit. Since the radiation incident on the radiation detectoris partially attenuated by a subject such as a body or an object through which the radiation has been transmitted, the radiation has a two-dimensional intensity distribution, and an image based on the generated electric signals also has a two-dimensional distribution corresponding to an internal structure of the subject.

Method for Manufacturing Radiation Detector

Next, a method for manufacturing the radiation detectorwill be described.is a flowchart illustrating the method for manufacturing the radiation detector, andis a schematic process cross-sectional view. The method for manufacturing the radiation detectoraccording to the first embodiment includes a step of forming a plurality of pixels (S), a step of acquiring inspection image data (S), a step of writing the image data into a storage device (S), and a step of mounting the storage device on a substrate (S). The method for manufacturing the radiation detectoraccording to the first embodiment may further include a step of forming scintillators (S).

A plurality of pixels each including a switching element and a photoelectric conversion element are formed on the substrate. As illustrated in, the substrateis prepared to form the pixel arraywith the first main surfaceincluding the plurality of pixels. To be specific, for example, a semiconductor manufacturing technique to be used for a liquid crystal display device is used to form the plurality of TFTsin the pixel regionof the first main surfaceof the substrate. Further, a plurality of the photodiodesrespectively connected to the plurality of TFTsare formed. Simultaneously of the formation of the pixel array, the plurality of scanning linesand data linesconnected to the TFTs, the padconnected to the scanning line, and the padconnected to the data lineare formed.

Thereafter, in a case where the substrateis an aggregate substrate corresponding to a plurality of the radiation detectors, the substrateis divided to have the size of the substrate of each of the radiation detectors.

The plurality of pixelsare irradiated with visible light to obtain inspection image data from the plurality of pixels. Such a step is executed as a part of a step of inspecting the pixelsfor a defect. First, as illustrated in, the pixel arrayis driven, a defect inspection device for the pixel arrayis prepared in order to detect generated charges, and a probe pinis brought into contact with the padand the pad. Next, the plurality of pixelsare irradiated with visible light, the pixelsare sequentially selected by applying a voltage to the pad, and the charges accumulated in the photodiode of the selected pixelsare read out by the defect inspection device via the pad. Example of the visible light to be irradiated may include white light.

The defect inspection device detects the read charge amount of each pixel as, for example, a voltage, performs A/D conversion to generate inspection image data of a luminance value, and temporarily stores such inspection image data. The quality of each pixel is determined based on the stored inspection image data. For example, if the luminance value is equal to or less than a predetermined value, the pixel is considered not to operate correctly, and thus, the defect inspection device determines that such a pixel is defective. The defect inspection device counts the number of pixels determined to be defective (hereinafter referred to as defective pixels), and if the number of defective pixels is smaller than a predetermined value, determines that the inspected radiation detectoris non-defective.

The inspection image data of the radiation detectordetermined to be non-defective is written into the storage device. For example, a writing device that can write data into the storage deviceis prepared, and the storage deviceis loaded into the writing device. The writing device receives the inspection image data of the radiation detectorfrom the defect inspection device and writes the received inspection image data into the storage device. The defect inspection device may temporarily store position information of the defective pixel, and the writing device may further write the stored position information of the defective pixel into the storage device.

The scintillatoris formed on the plurality of pixels. As described above, the scintillatoris bonded onto the plurality of pixelsusing the adhesive layersuch as OCA. Alternatively, the scintillatormay be formed by using a thin film formation technique.

The storage deviceinto which the image data is written is mounted on the substrate. The driver IC including the scanning line drive unitand the charge detection unitand the storage deviceare mounted in the peripheral regionof the substrate. Solder, conductive paste, or the like can be used for mounting. Thus, the radiation detectoris completed.

If necessary, the radiation detectoris incorporated into a housing. Thus, the radiation FPD is completed.

Imaging Method

Next, an imaging method according to the first embodiment will be described.is a schematic diagram illustrating a configuration of a radiation imaging systemused in an imaging method according to the first embodiment. The radiation imaging systemincludes the above-described radiation detector, a radiation source, a control device, and a monitor.is a block diagram illustrating a configuration example of the control device. First, the configuration of the radiation imaging systemwill be described.

The radiation sourceemits radiation passing through a subject. For example, the radiation sourceemits X-rays. The control deviceincludes a control circuit, a memory, a calibration unit, and an image processing unit. The memorystores the inspection image data stored in the storage deviceof the radiation detector. The memorystores captured image data obtained by detecting radiation passed through by the radiation detectorwhen the radiation is emitted from the radiation sourcetoward a subject. For example, the radiation detectorincludes pixels arranged in a matrix of r rows and s columns, and the luminance value of each pixel of the captured image data is represented by S(r=0, 1, 2, . . . , s=0, 1, 2).

The calibration unitreceives the inspection image data stored in the memoryand generates calibration data from the inspection image data. The inspection image data is configured by the luminance value of each pixel, and ideally, the luminance values of non-defective pixels are the same. However, due to variations in the thicknesses of the insulating layer and the semiconductor layer, the exposure condition of photolithography, the temperature condition of heat treatment, and the like at the time of manufacturing the radiation detector, variations also occur in the luminance value of each pixel. Thus, the use of the inspection image data is considered to make it possible to suppress the brightness unevenness in the captured image caused by such variations.

The variation in the luminance value of the inspection image data is considered to be proportional to the detected charge amount, and thus, for example, if such a variation is standardized to create calibration data and the captured image is corrected using such calibration data, the brightness unevenness in the captured image can be suppressed.

For example, the luminance value of each pixel of the inspection image data is represented by I. The calibration unitcalculates an average value Iof the luminance of all pixels of the inspection image data, and further obtains I/Ifor all the pixels. Such a value indicates the distribution of the sensitivity of each pixel, and thus, the inverse number of such a calculated value, that is, C=I, is calculated as the calibration coefficient C. If the storage devicealso stores the position information of the defective pixel, the calibration unitmay exclude the luminance value of the defective pixel in calculating the average value Iof the luminance of all the pixels.

The image processing unitreceives the captured image data Sfrom the memoryand receives the calibration coefficient Cfrom the calibration unit. The image processing unitmultiplies the captured image data Sby the calibration coefficient Cto generate calibrated captured image data S′. The generated calibrated captured image data S′is output to the monitor. The calibrated captured image data S′may be stored in the memory.

As described above, the brightness unevenness in the captured image data is caused by the process during manufacturing the active matrix substrate. Therefore, as long as the characteristics of the active matrix substratedo not change due to, for example, aged deterioration, brightness unevenness is considered to be appropriately suppressed by using the same calibration coefficient Cstored in the storage device. In this sense, it can be said that the calibration coefficient Cobtained from the inspection image data is used to calibrate the brightness unevenness unique to each active matrix substratein the captured image.

When the storage devicealso stores the position information of the defective pixel, the image processing unitmay obtain the luminance value of the defective pixel by interpolation. Specifically, values of the captured image data S′of the pixels surrounding the defective pixel, for example, four pixels adjacent in the row and column directions, of the calibrated captured image data S′, may be totaled and divided by four to evaluate the luminance value of the defective pixel.

Next, an imaging method using the radiation imaging systemwill be described.is a flowchart illustrating the imaging method according to the first embodiment. The imaging method according to the first embodiment includes a step of reading the inspection image data or the calibration parameter from the storage device (S), a step of capturing a subject with radiation (S), and a step of calibrating the image data of the subject (S).

First, in the radiation imaging system, the inspection image data Iis read out from the storage deviceof the radiation detectorand stored in the memoryof the control device. As described above, the inspection image data or the calibration parameter is acquired in advance by irradiating the plurality of pixels with visible light during manufacture of the radiation detector. Such inspection image data is used to determine the defect of each pixel.

Subsequently, the calibration unitevaluates the calibration coefficient Cusing the inspection image data.

The radiation detectoris arranged so as to be positioned behind a subject with respect to the radiation source, and the radiation is emitted from the radiation sourcetoward the subject. The radiation detectordetects the radiation transmitted through the subject, and the obtained image data of the subject is transmitted to the control device.